US intelligence chiefs have confirmed that the National Security Agency has used a "back door" in surveillance law to perform warrantless searches on Americans’ communications.

The NSA's collection programs are ostensibly targeted at foreigners, but in August the Guardian revealed a secret rule change allowing NSA analysts to search for Americans' details within the databases.

Now, in a letter to Senator Ron Wyden, an Oregon Democrat on the intelligence committee, the director of national intelligence, James Clapper, has confirmed for the first time the use of this legal authority to search for data related to “US persons”.

“There have been queries, using US person identifiers, of communications lawfully acquired to obtain foreign intelligence targeting non-US persons reasonably believed to be located outside the United States,” Clapper wrote in the letter, which has been obtained by the Guardian.

“These queries were performed pursuant to minimization procedures approved by the Fisa court and consistent with the statute and the fourth amendment.”

The legal authority to perform the searches, revealed in top-secret NSA documents provided to the Guardian by Edward Snowden, was denounced by Wyden as a “backdoor search loophole.”